Vivid Unit + Vivid Unit Extender + GPSSDR. After assembling I was able to interact with the desktop for a few minutes while attempting to set up a wifi connection when unit powered off. Since then I'm only able to boot far enough to see the desktop appear before it powers off again. I've removed the extender board and GPSSDR to interact with the Vivid Unit by itself, and the same, it achieves the desktop and promptly powers off. I'm using the USB-C 20W power recharger I use with my iPad. Not sure where to go from here.
I allowed the unit to sit for an hour, then it booted up normally and I was able to interact with it for several minutes before it powered off once more. I don't want to attempt loading new software if it's likely to crash. Might this be an over-heat of the CPU?
Using a USB-C port on my iMac provided the correct flavor of power to run the Vivid Unit properly, but then I ran into the lack of the installed default administrative password for Debian and found I couldn't do much of anything without it, but the unit does seem to be running correctly now.
The issue is the power supply couldn't deliver enough current, and there are already several topics for this issue in the forum.
You may read them first, or just read the "Why does the Vivid Unit randomly shut down when starting/running GPSDR++?" part in the FAQ on product page.
By default you login as "vivid" user (with password "unit", source here), and you can use sudo to increase previlege without giving a password.